Raman amplifier and optical communication system including the same

ABSTRACT

The present invention relates to a Raman amplifier where flexibility in device design considering both of Raman amplification and dispersion compensation is high. In the Raman amplifier, the Raman amplification optical fiber included in the optical amplification section and the dispersion compensating optical fiber included in the dispersion compensation section are arranged while being optically connected to each other. Since the optical amplification section and the dispersion compensation section are provided as independent optical devices, one device can be designed without being restricted to the design conditions of the other device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Raman amplifier Raman-amplifyingsignal light of a plurality of channels having wavelengths differentfrom each other, and an optical communication system including the same.

2. Related Background Art

A rare earth element-doped optical fiber amplifier which uses a rareearth element-doped optical fiber as an optical amplification medium isan optical device having a structure for supplying pumping light havinga wavelength to pump the rare earth element into the optical fiber, andamplifying signal light by using the transition between the energylevels of the rare earth element. Therefore, in a rare earthelement-doped optical fiber amplifier, the wavelength band range ofsignal light which can be amplified is limited. Whereas the Ramanamplifier is an optical amplifier using the Raman scattering phenomenain an optical fiber where signal light propagates, and if thetransmission medium of the signal light is a silica-based optical fiber,then the signal light can be Raman-amplified by supplying pumping light,having a wavelength about 100 nm shorter than the signal lightwavelength, to the optical fiber. Therefore, with the Raman amplifier,the wavelength band range of signal light which can be amplified isarbitrary, and the pumping light wavelength can be appropriately setaccording to the signal light wavelength.

As the Raman amplifier, not only a structure for Raman-amplifying signallight in an optical fiber transmission line laid in the relay section,but also a structure, as a module provided in a repeater, forRaman-amplifying signal light in the repeater is known. The Ramanamplifier is an optical device using Raman scattering, which is one typeof non-linear optical phenomena in a Raman amplification optical fiber.Since a dispersion compensating optical fiber compensates for achromatic dispersion of the optical fiber transmission line is, ingeneral, an optical fiber having a small effective area and a highnon-linearity, a structure for Raman-amplifying signal light by usingthis dispersion compensating optical fiber as a Raman amplificationoptical fiber is also known.

SUMMARY OF THE INVENTION

The present inventors studied conventional Raman amplifiers, anddiscovered the following problems. In the case of a Raman amplifierwhere the dispersion compensating optical fiber is applied as a Ramanamplification optical fiber, it is necessary that one optical fiberrealizes both of the Raman amplification function and the dispersioncompensation function, so one function is restricted by the otherfunction. For example, in order to compensate for a chromatic dispersionof the optical fiber transmission line, the length of a dispersioncompensating optical fiber is set according to not only the chromaticdispersion and the length of the optical fiber transmission line, butalso according to the chromatic dispersion of the dispersioncompensating optical fiber itself. But, if the dispersion compensatingoptical fiber for which the length is set like this is applied to theRaman amplifier as a Raman amplification optical fiber, a sufficientRaman amplification gain may not be obtained. Therefore, in aconventional Raman amplifier, the design flexibility thereof is low forboth of the device design considering Raman amplification and the devicedesign considering dispersion compensation.

It is an object of the present invention to provide a Raman amplifierhaving high design flexibility for both of the device design consideringRaman amplification and the device design considering dispersioncompensation, and an optical communication system including the same.

The Raman amplifier according to the present invention is an opticaldevice for Raman-amplifying signal light (WDM signal light) of aplurality of channels having wavelengths different from each other,which is provided at a predetermined position on an optical fibertransmission line for capturing signal light propagating the opticalfiber transmission line and has an input end, and an output end foroutputting Raman-amplified signal light. Particularly, the Ramanamplifier according to the present invention comprises a lightamplification section and a dispersion compensation section, which areprovided between the input end and the output end respectively whilebeing optically connected to each other. The optical amplificationsection includes a Raman amplification optical fiber forRaman-amplifying the signal light by supplied pumping light. Thedispersion compensation section includes a dispersion compensatingoptical fiber, for example, and compensates for a chromatic dispersionof the optical fiber transmission line and the Raman amplificationoptical fiber in a signal light wavelength band.

The pumping light may be the pumping light of a plurality of channelshaving wavelengths different from each other, so as to enable Ramanamplification with a wider signal light wavelength band.

In the Raman amplifier according to the present invention, it ispreferable that the signal light propagation path from the input end tothe output end, excluding the dispersion compensation section, has acumulative chromatic dispersion whose absolute value is 5 ps/nm or lessin the signal light wavelength band. In this case, the dispersioncompensation section is designed such that the optical fibertransmission line, positioned outside the Raman amplification section,becomes the dispersion compensation target.

The Raman amplifier according to the present invention further comprisesa pumping light supply system for supplying pumping light having atleast a sufficient power to cause induced Raman scattering to the Ramanamplification optical fiber. This pumping light supply systemconstitutes a part of the light amplification section of the Ramanamplifier, and includes a pumping light source (first pumping lightsource) for supplying pumping light to the Raman amplification opticalfiber, and a first optical multiplexing structure for guiding thepumping light from the first pumping light source to the Ramanamplification optical fiber without passing through the dispersioncompensation section, such as a dispersion compensating optical fiber.

As described above, the Raman amplifier according to the presentinvention has a dispersion compensation section for implementing thedispersion compensation function and a light amplification section forimplementing the Raman amplification function, which are provided asoptical devices independent from each other, so high flexibility isobtained for both of the device design considering Raman amplificationand the device design considering dispersion compensation. Specifically,when the signal light propagation path from the input end to the outputend, excluding the dispersion compensation section in the Ramanamplifier, has a cumulative chromatic dispersion whose absolute value is5 ps/nm or less in the signal light wavelength band, it is substantiallyunnecessary for the dispersion compensation section to compensate forthe chromatic dispersion in the Raman amplification optical fiber in theRaman amplifier, so an even higher design flexibility is obtained.

In the Raman amplifier according to the present invention, Ramanamplification can be performed in the dispersion compensation section ifthe dispersion compensation section has a configuration which includes adispersion compensating optical fiber. In this case, it is preferablethat the pumping light supply system includes a pumping light source(second pumping light source) for supplying pumping light having asufficient power to cause induced Raman scattering to the dispersioncompensating optical fiber, and a second optical multiplexing structurefor guiding the pumping light from the pumping light source to thedispersion compensating optical fiber without passing through the Ramanamplification optical fiber. These first and second pumping lightsources may be a common pumping light source. Raman amplification isalso possible in the dispersion compensating optical fiber by installingthe dispersion compensating optical fiber at a position where thepumping light which propagated at least a part of the Ramanamplification optical fiber reaches.

In particular, Raman amplification in the dispersion compensatingoptical fiber can make the dispersion compensating optical fiber to besubstantially no loss in the signal light wavelength band. In otherwords, it is preferable that the pumping light to be supplied to thedispersion compensating optical has a sufficient power or more to obtainRaman gain to cancel transmission loss in the dispersion compensatingoptical fiber. In other words, the signal light is Raman-amplified alsoin an part excluding the light amplification section (dispersioncompensation section), so an effective loss of the dispersioncompensation section in the signal light wavelength band decreases, andthe loss becomes substantially none. In this case, the effective gain ofRaman amplification in the Raman amplifier is roughly the same as theRaman amplification gain in the light amplification section, so theflexibility of the device design considering both Raman amplificationand dispersion compensation further increases, and a deterioration ofnoise figure is effectively controlled.

In the Raman amplifier according to the present invention, the Ramanamplification optical fiber may include a forward stage Ramanamplification optical fiber provided at the upstream side and a backwardstage Raman amplification optical fiber provided at the downstream side,in view from the signal light propagation direction. In this case, theRaman amplifier Raman-amplifies the signal light in both of the forwardstage Raman amplification optical fiber and the backward stage Ramanamplification optical fiber, so the signal light can be Raman-amplifiedat low noise and high gain. In particular, it is preferable that thedispersion compensation section is arranged between the forward stageRaman amplification optical fiber and the backward stage Ramanamplification optical fiber to effectively control the deterioration ofnoise figure characteristic.

In the Raman amplifier according to the present invention, the Ramanamplification optical fiber may have a chromatic dispersion whoseabsolute value is 5 ps/nm/km or more in the signal light wavelengthband, or may have a zero dispersion wavelength of shorter than theshortest wavelength of the pumping light to be supplied. In this case,the generation of four wave mixing (including remote four wave mixing)is effectively controlled, and an excellent Raman amplificationcharacteristic is obtained. Particularly, when such Raman amplificationoptical fiber comprises the forward stage and backward stage opticalfibers, the signal light propagation path from the input end to theoutput end in the Raman amplifier, excluding the dispersion compensationsection, can have a cumulative chromatic dispersion whose absolute valueof which is 5 ps/nm or less in the signal light wavelength band, if thepolarity of the chromatic dispersion of the forward stage optical fiberand the polarity of the chromatic dispersion of the backward stageoptical fiber are set to be opposite.

Also in the Raman amplifier according to the present invention, it ispreferable that the Raman amplification optical fiber has an effectivearea of 30 μm² or less at a pumping light wavelength. This is becausethe Raman gain coefficient increases and high efficiency Ramanamplification becomes possible. In the Raman amplifier according to thepresent invention, it is preferable that the Raman amplification opticalfiber has a cut-off wavelength of shorter than the shortest wavelengthof the pumping light to be supplied. This is because the pumping lightto be supplied to the Raman amplification optical fiber propagates in asingle mode, so stable gain can be obtained. Also in the Raman amplifieraccording to the present invention, it is preferable that the signallight propagation path from the input end to the output end is 1 ps orless in the signal light wavelength band. In this case, deterioration ofthe transmission characteristic is controlled for the Raman amplifier.

The pumping light supply system in the Raman amplifier according to thepresent invention may include a pumping light source for outputtingpumping light and a drive circuit for driving the pumping light source.The pumping light source and drive circuit may be provided separatelyfrom the optical amplification section, so that installation is possibleafter Raman amplification optical fibers are installed.

The optical communication system according to the present inventioncomprises an optical fiber transmission line where signal light of aplurality of channels propagate, and a Raman amplifier having the abovementioned structure. Particularly to enable a long haul transmission,the optical communication system according to the present invention maycomprises a plurality of Raman amplifiers each having a structuresimilar to the Raman amplifier.

In the optical communication system according to the present invention,various modifications to improve the SN ratio is possible to furtherimprove system performance. In other words, the optical communicationsystem according to the present invention may have a configuration tofurther improve the noise characteristic by causing induced Ramanscattering in the optical fiber transmission line at the input end sideof the Raman amplifier. Specifically, the optical communication systemmay comprise a pumping light source (third pumping light source) forsupplying new pumping light to the optical fiber transmission line atthe input end side, and a third optical multiplexing structure forguiding the pumping light from the pumping light source to the opticalfiber transmission line. In the case of a configuration where aplurality of Raman amplifiers are provided on an optical fibertransmission line, it is efficient to supply the pumping light to theoptical fiber transmission line at the input end side of the Ramanamplifier which locates at the most upstream side among the plurality ofRaman amplifiers. Also this optical communication system may comprise abypass transmission line for supplying pumping light, which propagatesthrough at least a part of the Raman amplification optical fiber of theRaman amplifier, to the optical fiber transmission line at the input endside of the Raman amplifier, and a fourth optical multiplexing structurefor guiding the pumping light, which propagates through the bypasstransmission line, to the optical fiber transmission line. In this case,it is preferable that the Raman amplifier includes an opticaldemultiplexer for guiding the light propagated through the Ramanamplification optical fiber to the bypass transmission line, and anoptical filter for transmitting the pumping light out of thedemultiplexed lights by the optical demultiplexer.

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not to beconsidered as limiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting the configuration of the first embodimentof the Raman amplifier according to the present invention;

FIG. 2 is a diagram depicting the configuration of the second embodimentof the Raman amplifier according to the present invention;

FIG. 3 is a diagram depicting the configuration of the Raman amplifierof the first comparison example;

FIG. 4 is a diagram depicting the configuration of the Raman amplifierof the second comparison example;

FIG. 5A and FIG. 5B are graphs depicting the gain characteristic and thenoise figure characteristic of the first embodiment, second embodiment,first comparison example, and second comparison example respectively;

FIG. 6 is a table showing the output power of each semiconductor laserlight source (pumping light source) of the Raman amplifiers of the firstembodiment, second embodiment, first comparison example, and secondcomparison example respectively;

FIG. 7 is a graph depicting the relationship between the relativerefractive index difference and g_(R) in the core region of the opticalfiber;

FIG. 8 is a graph depicting the relationship between the relativerefractive index difference and transmission loss α in the core regionof the optical fiber;

FIG. 9 is a diagram depicting a general configuration of the Er elementadded optical fiber amplifier;

FIG. 10 is a graph depicting the relationship between the signal lightoutput power and the power penalty per channel of the Raman amplifier;

FIG. 11 is a diagram depicting the configuration of the first embodimentof the optical communication system according to the present invention;and

FIG. 12A and FIG. 12B are diagrams depicting the configuration of thesecond embodiment of the optical communication system according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the Raman amplifier and the optical communication systemaccording to the present invention will now be described with referenceto the FIGS. 1-4, 5A-5B, 6-11, and 12A and 12B. In these drawings, thesame elements are denoted with the same symbols, where redundantdescriptions are omitted.

First Embodiment of Raman Amplifier

FIG. 1 is a diagram depicting the configuration of the first embodimentof the Raman amplifier according to the present invention. The Ramanamplifier 1 according to the first embodiment comprises an opticalisolator 111, a Raman amplification optical fiber 121, an opticalcoupler 141 (included in the first optical multiplexing structure), adispersion compensating optical fiber 132, an optical coupler 142(included in the second optical multiplexing structure), and an opticalisolator 112, which are sequentially provided from the input end 101 tothe output end 102. The semiconductor laser light sources 161 a to 161 c(first pumping light source) are connected to the optical coupler 141through the optical multiplexer 151. The semiconductor laser lightsources 162 a to 162 c (second pumping light source) are connected tothe optical coupler 142 through the optical multiplexer 152. The opticalcouplers 141 and 142 (first and second optical multiplexing structures),the optical multiplexers 151 and 152, and the semiconductors laser lightsources 161 a to 161 c and 162 a to 162 c (first and second pumpinglight sources) constitute the pumping light supply system 100.

In the Raman amplifier 1 according to the first embodiment, the opticalamplification section includes the Raman amplification optical fiber121, the optical coupler 141, the optical multiplexer 151, and thesemiconductor laser light sources 161 a to 161 c. The dispersioncompensation section includes the dispersion compensating an opticalfiber 132, and pumping light is supplied from the semiconductor laserlight sources 162 a to 162 c is supplied to the dispersion compensatingoptical fiber 132 through the optical multiplexer 152 and the opticalcoupler 142.

The optical isolator 111 transmits the light reaching from the input end101 to the Raman amplification optical fiber 121, and blocks the lightwhich propagates in a direction opposite from this transmission light.The optical isolator 112 transmits the light reaching from the opticalcoupler 142 to the output end 102, and blocks the light which propagatesin a direction opposite from this transmission light. The Ramanamplification optical fiber 121 Raman-amplifies the signal light guidedfrom the optical isolator 111 by pumping light supplied from the opticalcoupler 141.

The dispersion compensating optical fiber 132 compensates for thechromatic dispersion in the signal light wavelength in the optical fibertransmission line where this Raman amplifier 1 is provided, and in theRaman amplification optical fiber 121. When the absolute value of thecumulative chromatic dispersion in the signal light propagation pathfrom the input end 101 to the output end 102, excluding the dispersioncompensating optical fiber 132, is 5 ps/nm or less in the signal lightwavelength band, the compensation target of the dispersion compensatingoptical fiber 132 becomes the chromatic dispersion in the signal lightwavelength of the optical fiber transmission line where the Ramanamplifier 1 is provided.

The semiconductor laser light sources 161 a to 161 c output laser beamswith different wavelengths respectively. The optical multiplexer 151multiplexes the laser beams which were output from the semiconductorlaser light sources 161 a to 161 c respectively, and outputs themultiplexed light to the optical coupler 141 as the pumping light of aplurality of channels. The optical coupler 141 directly guides thepumping light reaching from the optical multiplexer 151 to the Ramanamplification optical fiber 121, and also guides the signal light of theplurality of channels reaching from the Raman amplification opticalfiber 121 to the dispersion compensating optical fiber 132.

The semiconductor laser light sources 162 a to 162 c output laser beamswith different wavelengths respectively. The optical multiplexer 152multiplexes the laser beams which were output from the semiconductorlaser light sources 162 a to 162 c respectively, and outputs themultiplexed light to the optical coupler 142 as the pumping light of themultiple channels. The optical coupler 142 directly guides the pumpinglight reaching from the optical multiplexer 152 to the dispersioncompensating optical fiber 132, and outputs the signal light reachingfrom the dispersion compensating optical fiber 132 to the opticalisolator 112.

For example, signal light to be Raman-amplified is a WDM signal in the Sband (wavelength band range of 1460 nm to 1530 nm), the outputwavelength (pumping channel wavelength) of the semiconductor laser lightsources 161 a and 162 a respectively is 1390 nm, the output wavelength(pumping channel wavelength) of the semiconductor laser light sources161 b and 162 b is 1405 nm respectively, and the output wavelength(pumping channel wavelength) of the semiconductor laser light sources161 c and 162 c respectively is 1430 nm.

The optical transmission line is, for example, a single mode opticalfiber which has a zero dispersion wavelength near the wavelength of 1.3μm, and has a positive chromatic dispersion in the signal lightwavelength band. For the Raman amplification optical fiber 121, anoptical fiber having a small effective area and high non-linearity issuitable, and specifically, high Raman amplification efficiency can beobtained if the effective area thereof is 30 μm² or less at the pumpinglight wavelength.

The Raman amplification optical fiber 121 may have a chromaticdispersion whose absolute value is 5 ps/nm/km or more in the signallight wavelength band, and have a zero dispersion wavelength of shorterthan the shortest wavelength of the pumping light. In this case, thegeneration of four wave mixing (including remote four wave mixing) iseffectively controlled, and an excellent Raman amplificationcharacteristic can be obtained. It is preferable that the Ramanamplification optical fiber 121 has a cut-off wavelength of shorter thanthe shortest wavelength of the pumping light, and in this case, stablegain can be obtained since the pumping light propagates in the Ramanamplification optical fiber 121 in single mode.

It is preferable that the signal light propagation path from the inputend 101 to the output end 102 has a 1 ps or less polarization modedispersion in the signal light wavelength band. In this case,deterioration of the Raman amplification characteristic can beeffectively controlled.

The Raman amplifier 1 according to the first embodiment operates asfollows. The laser beams which were output from the semiconductor laserlight sources 161 a to 161 c respectively are multiplexed by the opticalmultiplexer 151, and the pumping light of the plurality of channels,which is the multiplexed laser beam, is supplied to the Ramanamplification optical fiber 121 through the optical coupler 141. Thelaser beams which were output from the semiconductor laser light sources162 a to 162 c respectively are multiplexed by the optical multiplexer152, and the pumping light of the plurality of channels, which is themultiplexed laser beam, is supplied to the dispersion compensatingoptical fiber 132 through the optical coupler 142. The signal light ofthe plurality of channels entered from the input end 101 reach the Ramanamplification optical fiber 121 through the optical isolator 111, and isRaman-amplified in the Raman amplification optical fiber 121.

The signal light which was Raman-amplified in the Raman amplificationoptical fiber 121 passes through the optical coupler 141 and reaches thedispersion compensating optical fiber 132, and is furtherRaman-amplified in the dispersion compensating optical fiber 132. Thesignal light which was Raman-amplified in the dispersion compensatingoptical fiber 132 then passes through the optical coupler 142 and theoptical isolator 112 sequentially, and is output from the output end 102to the optical fiber transmission line outside. The dispersioncompensating optical fiber 132 not only Raman-amplifies the signallight, but also functions so as to compensate for the chromaticdispersion in the signal light wavelength of the optical fibertransmission line and Raman amplification optical fiber 121.

Therefore the Raman amplifier 1 according to the first embodimentprovides high flexibility to the device design considering both Ramanamplification and dispersion compensation. In other words, the loss ofsignal light which propagates through the optical fiber transmissionline is compensated by the Raman amplification in the Ramanamplification optical fiber 121 of this Raman amplifier 1. The chromaticdispersion in the signal light wavelength of the optical fibertransmission line and the Raman amplification optical fiber 121, on theother hand, is compensated by the dispersion compensating optical fiber132 in the Raman amplifier 1. Since the dispersion compensating opticalfiber 132 which implements the dispersion compensation function, and theRaman amplification optical fiber 121 which implements the Ramanamplification function, are optically connected in this way, the Ramanamplifier 1 can provide high design flexibility for both Ramanamplification and dispersion compensation.

In the Raman amplifier 1 according to the first embodiment, signal lightis Raman-amplified not only in the Raman amplification optical fiber121, but also in the dispersion compensating optical fiber 132, so thedispersion compensating optical fiber 132 has less effective loss in thesignal light wavelength, and becomes a transmission medium withsubstantially loss-less. In this case, the effective gain of Ramanamplification of the signal light in the Raman amplifier 1 is roughlythe same as the Raman amplification gain of the signal light in theRaman amplification optical fiber 121, so the design flexibility forboth Raman amplification and dispersion compensation further increases,and deterioration of the noise figure can also be effectivelycontrolled.

Second Embodiment of Raman Amplifier

FIG. 2 is a diagram depicting the configuration of the second embodimentof the Raman amplifier according to the present invention. The Ramanamplifier 2 according to the second embodiment is different from theRaman amplifier 1 according to the first embodiment in that a new Ramanamplification optical fiber 122 is provided between the dispersioncompensating optical fiber 132 and the optical coupler 142. Thedispersion compensating optical fiber 132 is arranged between theforward stage Raman amplification optical fiber 121 and the backwardstage Raman amplification optical fiber 122.

In the Raman amplifier 2 according to the second embodiment, the opticalamplification section comprises the forward stage Raman amplificationoptical fiber 121, the optical coupler 141 (first optical multiplexingstructure), an optical multiplexer 151, the semiconductor laser lightsources 161 a to 161 c (first pumping light source), the backward stageRaman amplification optical fiber 122, the optical coupler 142 (secondoptical multiplexing structure), the optical multiplexer 152, and thesemiconductor laser light sources 162 a to 162 c (second pumping lightsource). The optical couplers 141 and 142 (first and second opticalmultiplexing structures), the optical multiplexers 151 and 152, and thesemiconductor laser light sources 161 a to 161 c and 162 a to 162 c(first and second pumping light sources) constitute the pumping lightsupply system 100. The dispersion compensation section includes thedispersion compensating optical fiber 132, and pumping light, which wassupplied from the semiconductor laser light sources 162 a to 162 c tothe backward stage Raman amplification optical fiber 122 through theoptical multiplexer 152 and optical coupler 142, and which propagatedthrough the backward stage Raman amplification optical fiber 122, issupplied to this dispersion compensating optical fiber 132.

Pumping light, which was output from the semiconductor laser lightsources 161 a to 161 c (multiplexed light multiplexed by the opticalmultiplexer 151), is supplied to the forward stage Raman amplificationoptical fiber 121 through the optical fiber 141. This Ramanamplification optical fiber 121 Raman-amplifies the signal lightreaching from the optical isolator 111, and the Raman-amplified signallight is output to the optical coupler 141.

Pumping light, which was output from the semiconductor laser lightsources 162 a to 162 c (multiplexed light multiplexed by the opticalmultiplexer 152), is supplied to the backward stage Raman amplificationoptical fiber 122 through the optical coupler 142. This Ramanamplification optical fiber 122 Raman-amplifies the signal lightreaching from the dispersion compensating optical fiber 132, and theRaman-amplified signal light is output to the optical coupler 142.

It is preferable that the Raman amplification optical fibers 121 and 122are transmission medium having a small effective area and a highnon-linearity respectively, and specifically, the optical fibers foramplification 121 and 122 have an effective area of 30 μm² or less atthe pumping light wavelengths respectively, so as to obtain high Ramanamplification efficiency. It is also preferable that each one of theRaman amplification optical fibers 121 and 122 has a chromaticdispersion whose absolute value is 5 ps/nm/km or more in the signallight wavelength band, and a zero dispersion wavelength of shorter thanthe shortest wavelength of the pumping light, and in this case, thegeneration of four wave mixing (including remote four wave mixing) iseffectively controlled, and an excellent Raman amplificationcharacteristic is obtained. Also it is preferable that the Ramanamplification optical fibers 121 and 122 have a cut-off wavelength ofshorter than the shortest wavelength of the pumping light respectively,and in this case, stable gain can be obtained since the pumping lightpropagates in the Raman amplification optical fibers 121 and 122 insingle mode. It is preferable that the signal light propagation pathfrom the input end 201 to the output end 202 has a 1 ps or lesspolarization mode dispersion in the signal light wavelength band, and inthis case, deterioration of the Raman amplification characteristic iseffectively controlled.

Particularly, it is preferable that the signal light propagation pathfrom the input end 201 to the output end 202, excluding the dispersioncompensating optical fiber 132, has a cumulative chromatic dispersionwhose absolute value is 5 ps/nm or less in the signal light wavelengthband. Therefore it is preferable that the Raman amplification opticalfibers 121 and 122 have a chromatic dispersion with different signs inthe signal light wavelength band respectively. In this case, thecompensation target of the dispersion compensating optical fiber 132 isthe optical fiber transmission line where this Raman amplifier 2 isinserted, and the chromatic dispersion in the signal light wavelength ofthe optical fiber transmission line is compensated for.

The Raman amplifier 2 according to the second embodiment operates asfollows. The laser beams which were output from the semiconductor lasersources 161 a to 161 c respectively are multiplexed by the opticalmultiplexer 151, and the pumping light of a plurality of channels, whichis the multiplexed laser beam, is supplied to the Raman amplificationoptical fiber 121 through the optical coupler 141. The laser beams whichwere output from the semiconductor laser light sources 162 a to 162 crespectively are multiplexed by the optical multiplexer 152, and thepumping light of the plurality of channels, which is the multiplexedlaser beam, is sequentially supplied to the Raman amplification opticalfiber 122 and the dispersion compensating optical fiber 132 through theoptical coupler 142.

The signal light of the plurality of channels entering from the inputend 201 passes through the optical isolator 111, reaches the Ramanamplification optical fiber 121, and is Raman-amplified in the Ramanamplification optical fiber 121. The signal light which wasRaman-amplified in the Raman amplification optical fiber 121 passesthrough the optical coupler 141 and reaches the dispersion compensatingoptical fiber 132 and the Raman amplification optical fiber 122sequentially, and is Raman-amplified also in both the optical fibers 132and 122. The signal light which was Raman-amplified in the Ramanamplification optical fiber 122 passes through the optical coupler 142and the optical isolator 112 sequentially, and is output from the outputend 202 to the optical fiber transmission line outside. Also thedispersion compensating optical fiber 132 not only Raman-amplifies thesignal light, but also functions so as to compensate for the chromaticdispersion in the signal light wavelength of the optical fibertransmission line and the Raman amplification optical fibers 121 and122.

Therefore the Raman amplifier 2 according to the second embodimentprovides high design flexibility for both Raman amplification anddispersion compensation, just like the case of the first embodiment. Inother words, the loss of the signal light, which propagates through theoptical fiber transmission line, is compensated by the Ramanamplification in the Raman amplification optical fibers 121 and 122 ofthis Raman amplifier 2. The chromatic dispersion in the signal lightwavelength of the optical fiber transmission line and the Ramanamplification optical fibers 121 and 122, on the other hand, iscompensated for by the dispersion compensating optical fiber 132 in theRaman amplifier 2. Since the dispersion compensating optical fiber 132,which implements the dispersion compensation function, and the Ramanamplification optical fibers 121 and 122, which implement the Ramanamplification functions, are provided while being optically connected toeach other, the Raman amplifier 2 can provide high design flexibilityfor both Raman amplification and dispersion compensation.

In the Raman amplifier 2 according to the second embodiment, the signallight is Raman-amplified not only in the Raman amplification opticalfibers 121 and 122, but also in the dispersion compensating opticalfiber 132, so the dispersion compensating optical fiber 132 has lesseffective loss in the signal light wavelength, and becomes atransmission medium with substantially no loss. In this case, theeffective gain of Raman amplification of the signal light in the Ramanamplifier 2 is roughly the same as the Raman amplification gain of thesignal light in the Raman amplification optical fibers 121 and 122, sothe design flexibility for both Raman amplification and dispersioncompensation further increases, and deterioration of the noise figurecan also be effectively controlled.

Also if the signal light propagation path from the input end 201 to theoutput end 202, excluding the dispersion compensating optical fiber 132,is designed so as to have a cumulative chromatic dispersion whoseabsolute value is 5 ps/nm or less at the signal light wavelength band inthe Raman amplifier 2 according to the second embodiment, then thedispersion compensating optical fiber 132 compensates for the chromaticdispersion of the optical fiber transmission line at the signal lightwavelength, with the optical fiber transmission line where this Ramanamplifier 2 is provided as a target to be compensated for. Thereforethis Raman amplifier 2 has high design flexibility for both Ramanamplification and dispersion compensation, and effectively controls thegeneration of a non-linear optical phenomena, such as self phasemodulation. Also in the Raman amplifier 2 according to the secondembodiment, the dispersion compensating optical fiber 132 is arrangedbetween the forward stage Raman amplification optical fiber 121 and thebackward stage Raman amplification optical fiber 122, so Ramanamplification with low noise and high gain becomes possible.

Comparison Example

Two comparison examples of the Raman amplifier according to the presentinvention will now be described.

FIG. 3 is a diagram depicting the configuration of the Raman amplifier 3according to the first comparison example. The difference between theRaman amplifier 3 according to the first comparison example and theRaman amplifier 1 according to the first embodiment is that thedispersion compensating optical fiber 131 is provided instead of theRaman amplification optical fiber 121. The Raman amplifier 3 accordingto this first embodiment has only the dispersion compensating opticalfibers 131 and 132, which implement the dispersion compensationfunction, and does not have the Raman amplification optical fiber forimplementing the Raman amplification function. In this first comparisonexample, both of the dispersion compensating optical fibers 131 and 132function as the Raman amplification optical fibers, and also function asthe dispersion compensation section for compensating for the chromaticdispersion of the optical fiber transmission line.

The Raman amplifier 3 according to this first comparison exampleoperates as follows. The signal light of a plurality of channelsentering from the input end 301 passes through the optical isolator 111,reaches the dispersion compensating optical fiber 131, and isRaman-amplified in the dispersion compensation optical fiber 131. Thesignal light, which was Raman-amplified in the dispersion compensatingoptical fiber 131, passes through the optical coupler 141, reaches thedispersion compensation optical fiber 132, and is also Raman-amplifiedin this dispersion compensating optical fiber 132. And the signal light,which was Raman-amplified in the dispersion compensating optical fiber132, passes through the optical coupler 142 and the optical isolator 112sequentially, and is output from the output end 302 to the optical fibertransmission line outside. The dispersion compensating optical fibers131 and 132 not only Raman-amplify the signal light, but also functionso as to compensate for the chromatic dispersion of the optical fibertransmission line at the signal light wavelength.

FIG. 4 is a diagram depicting the configuration of the Raman amplifier 4according to the second comparison example. The difference between theRaman amplifier 4 according to the second comparison example and theRaman amplifier 1 according to the first embodiment is that the Ramanamplification optical fiber 121, the optical the coupler 141, theoptical multiplexer 151, and the semiconductor laser light sources 161 ato 161 c, are not provided. In the Raman amplifier 4 according to thesecond comparison example, the dispersion compensating optical fiber 132implements the dispersion compensation function and the Ramanamplification function, and the Raman amplification optical fiber is notprovided.

The Raman amplifier 4 according to this second comparison exampleoperates as follows. The signal light of a plurality of channelsentering from the input end 401 passes through the optical isolator 111,reaches the dispersion compensating optical fiber 132, and isRaman-amplified in this dispersion compensating optical fiber 132. Thesignal light, which was Raman-amplified in the dispersion compensatingoptical fiber 132, passes through the optical coupler 142 and theoptical isolator 112 sequentially, and is output from the output end 402to the optical fiber transmission line outside. The dispersioncompensating optical fiber 132 not only Raman-amplifies the signallight, but also functions so as to compensate for the chromaticdispersion of the optical fiber transmission line at the signal lightwavelength.

Comparison of First and Second Embodiments, and First and SecondComparison Examples

Now the Raman amplifier 1 according to the first embodiment (FIG. 1),Raman amplifier 2 according to the second embodiment (FIG. 2), Ramanamplifier 3 according to the first comparison example (FIG. 3), and theRaman amplifier 4 according to the second comparison example will becompared with each other.

In the Raman amplifiers 1 and 2, the length of the Raman amplificationoptical fiber 121 is 3 km, the length of the Raman amplification opticalfiber 122 is 3 km, and the length of the dispersion compensating opticalfiber 132 is 15 km respectively. In the Raman amplifier 3, the length ofthe dispersion compensating optical fiber 131 is 3 km, and the length ofthe dispersion compensating optical fiber 132 is 12 km. And in the Ramanamplifier 4, the length of the dispersion compensating optical fiber 132is 15 km.

In this way, in each one of the Raman amplifiers 1 to 4, the totallength of the dispersion compensating optical fiber is set so as tomatch 15 km.

In each one of the Raman amplifiers 1 to 4, the insertion loss of theoptical isolator 111 is 1 dB, the insertion loss of the optical coupler141 is 0.6 dB, and the insertion loss of both the optical coupler 142and optical isolator 112 is 1.2 dB. In the Raman amplifier 2, theconnection loss of the dispersion compensating optical fiber 132 andRaman amplification optical fiber 122 is 0.3 dB. And in each one of theRaman amplifiers 1 to 4, the output power of each semiconductor laserlight source is set such that the average gain in the S band becomes 20dB.

FIGS. 5A and 5B are graphs depicting the gain characteristic and noisefigure characteristic of the Raman amplifiers 1 to 4 respectively. InFIG. 5A, the graph Gain 1 indicates the gain characteristic of the Ramanamplifier 1, graph Gain 2 indicates the gain characteristic of the Ramanamplifier 2, graph Gain 3 indicates the gain characteristic of the Ramanamplifier 3, and graph Gain 4 indicates the gain characteristic of theRaman amplifier 4 respectively. In FIG. 5B, graph NF1 indicates thenoise figure characteristic of the Raman amplifier 1, graph NF2indicates the noise figure characteristic of the Raman amplifier 2,graph NF3 indicates the noise figure characteristic of the Ramanamplifier 3, and graph NF4 indicates the noise figure characteristic ofthe Raman amplifier 4 respectively. As FIGS. 5A and 5B show, the noisefigures of the Raman amplifiers 1 and 2 according to the first andsecond embodiments are lower than the noise figures of the Ramanamplifiers 3 and 4 according to the first and second comparison examplesrespectively.

FIG. 6 is a table showing the output power of each semiconductor laserlight source of the Raman amplifiers 1 to 4 respectively. As this tableshows, the required pumping light power of the Raman amplifiers 1 and 2according to the first and second embodiments are lower than therequired pumping light power of the Raman amplifiers 3 and 4 accordingto the first and second comparison examples respectively.

Raman Amplifiers According to the First and Second Embodiments

The Raman amplifiers 1 and 2 (particularly the Raman amplificationoptical fibers 121 and 122) according to the first and secondembodiments will now be described.

Generally, compared with a rare earth element-doped optical fiberamplifier, the Raman amplifier has an advantage in that there is nolimit in the wavelength band that has gain, but there is a disadvantagein that the pumping efficiency is low. However the Raman gaincoefficient (g_(R)/A_(eff)) of the Raman amplification optical fibers121 and 122 can be increased by decreasing the effective area A_(eff) ofthe Raman amplification optical fibers 121 and 122.

FIG. 7 is a graph depicting the relationship between the relativerefractive index difference of the core region and g_(R). FIG. 7 showsthe relationship between the relative refractive index difference of thecore region and g_(R) for various optical fibers, such as a standardsingle mode optical fiber where GeO₂ is added to the core region, asingle mode optical fiber where the core region is pure silica glass andan F element is added to the cladding region, a dispersion-shiftedoptical fiber where the zero dispersion wavelength is shifted to thelonger wavelength side at wavelength 1.3 μm, a dispersion compensatingoptical fiber where the chromatic dispersion is negative at wavelength1.55 μm, and an optical fiber which effective area is small, andnon-linearity is high. As FIG. 7 shows, g_(R) is substantially in alinear relationship with the relative refractive index difference, andis 2.3×10⁻¹⁴ m/W or more. Therefore in the following description, it isassumed that g_(R)=2.3×10⁻¹⁴ m/W.

FIG. 8 is a graph depicting the relationship between the relativerefractive index difference of the core region and transmission loss α.In FIG. 8, graph L1 shows the relationship between the relativerefractive index difference of the core region in a typical opticalfiber and transmission loss α at wavelength 1.45 μm, and graph L2 showsthe relationship between the relative refractive index difference of thecore region in a typical optical fiber and transmission loss α at thewavelength 1.55 μm. According to FIG. 8, the transmission loss α at thepumping light wavelength is assumed to be 0.55 dB/km, and the actuallength L of the Raman amplification optical fiber, where the effectivelength L_(eff) of the Raman amplification optical fiber does not become½ or less of the actual length, is assumed to be 14.5 km.

The power pump P_(pump) of the pumping light to be supplied to theoptical fiber for Ramon amplification is assumed to be 500 mW, which isequivalent to the maximum input pumping light power to the optical fiberfor amplification with a general configuration of the Er-doped opticalfiber amplifier, which is commercialized as a centralized opticalamplifier.

The Er-doped optical fiber amplifier 9 with a general configurationcomprises, for example, an optical isolator 911, optical coupler 941,Er-doped optical fiber 931, optical isolator 912, optical coupler 942 a,Er-doped optical fiber 932, optical coupler 942 b, dispersioncompensator 971, optical isolator 913, optical coupler 943 a, Er-dopedoptical fiber 933, and optical coupler 943 b, which are arrangedsequentially from input end 901 towards output end 902, as shown in FIG.9. The pumping light, which is output from the semiconductor laser lightsource 961, is supplied to the Er-doped optical fiber 931 through theoptical coupler 941 in the forward direction with respect to the signallight. The pumping light, which is output from the semiconductor laserlight source 962, is branched into two by the optical branching unit952. One of the branched lights is supplied to the Er-doped opticalfiber 932 through the optical coupler 942 a in the forward directionwith respect to the signal light. The other branched light is suppliedto the Er-doped optical fiber 932 through the optical coupler 942 b inthe back direction with respect to the signal light. The pumping light,which is output from the semiconductor laser light source 963, issupplied to the Er-doped optical fiber 933 through the optical coupler943 a in the forward direction with respect to the signal light. Thepumping light, which is output from the semiconductor laser lightsources 964 a and 964 b respectively, is multiplexed by the opticalmultiplexer 954. And this multiplexed light is supplied to the Er-dopedoptical fiber 933 through the optical coupler 943 b in the backwarddirection with respect to the signal light.

If attenuation of the pumping light is ignored, then the Raman amplifiergain G_(Raman) (dB) is given by the following formula.$G_{Raman} = {{10 \cdot \log}\left\{ {\exp\left( {\frac{g_{R}}{A_{eff}}L_{eff}P_{pump}} \right)} \right\}}$

As a consequence, if the effective area A_(eff) of the Ramanamplification optical fiber is 30 μm² or less at the pumping lightwavelength, then the absolute value of the Raman amplification gainG_(Raman) becomes a loss of 25 dB or more per span (one relay section)in a typical land optical communication system, which is desirable.

Since the Raman amplification optical fiber is long, the wavelengthdeterioration of signal light tends to occur in Raman amplificationoptical fiber due to a non-linear optical phenomena, such as self phasemodulation and four wave mixing, if the effective area A_(eff) is small.However, in the case of the Raman amplifier according to the presentinvention, each of the chromatic dispersion and polarization modedispersion of the Raman amplification optical fiber is appropriatelyset, so wave form deterioration of the signal light is effectivelycontrolled.

If the Raman amplifier is applied to the optical communication system asa preamplifier, the loss of the optical demultiplexer, arranged betweenthe Raman amplifier as a pre-amplifier and the light receiving section,is generally about 10 dB, and the light receiving dynamic range of thelight receiving section per channel is generally −16 dBm/ch to −10dBm/ch. Therefore the signal light output power per channel of the Ramanamplifier requires −6 dBm/ch or more.

FIG. 10 is a graph depicting the relationship between the signal lightoutput power per channel of the Raman amplifier and power penalty. Here8 channels of multiplexed signal light are input to the Raman amplifier.The Raman amplification optical fiber has high non-linearity. In FIG.10, graph P1 shows the relationship in the Raman amplification opticalfiber with the chromatic dispersion of +2 ps/nm/km, graph P2 shows therelationship in the Raman amplification optical fiber with the chromaticdispersion of −2 ps/nm/km, graph P3 shows the relationship in the Ramanamplification optical fiber with the chromatic dispersion of −5ps/nm/km, graph P4 shows the relationship in the Raman amplificationoptical fiber with the chromatic dispersion of −7 ps/nm/km, graph P5shows the relationship in the Raman amplification optical fiber with thechromatic dispersion of −20 ps/nm/km, and graph P6 shows therelationship in the Raman amplification optical fiber with the chromaticdispersion of −40 ps/nm/km. As FIG. 10 shows, if the Raman amplificationoptical fiber has high non-linearity and has a chromatic dispersionwhose absolute value is 5 ps/nm/km or less, then the power penaltycaused by the four wave mixing does not become 1 dB or less unless thesignal light output power per channel is 2 dBm or less. When the resultis applied to the case of 64 channel signal light transmission, thesignal light output power per channel is −7 dBm, which is outside thelight receiving dynamic range of the light receiving section in theoptical communication system where the Raman amplifier is applied to thepreamplifier. However, as mentioned above, this problem can be avoidedif each one of the Raman amplification optical fibers 121 and 122 has achromatic dispersion whose absolute value is 5 ps/nm/km or more in thesignal light wavelength band.

For example, it is preferable that the Raman amplification optical fiber121 is mainly made from silica glass, and comprises a GeO₂-doped coreregion having an outer diameter of 4.0 μm, and an F-doped claddingregion surrounding the core region, where the relative refractive indexdifference of the core region is +2.5%, and the relative refractiveindex difference of the cladding region is −0.7% with respect to thepure silica glass. If the refractive index of the pure silica glass isn₀, the refractive index of the core region is n₁, the refractive indexof the cladding region is n₂, then the relative refractive indexdifference Δ₁ of the core region and the relative refractive indexdifference Δ₂ of the cladding region with respect to the pure silicaglass are given by the following formulas respectively.Δ₁=(n ₁ ² −n ₀ ²)/2n ₀ ²Δ₂=(n ₂ ² −n ₀ ²)/2n ₀ ²In this case, the Raman amplification optical fiber 121 has a chromaticdispersion of −9.0 ps/nm/km at a wavelength of 1.55 μm, an effectivearea of 9.9 μm², and a Raman gain coefficient of 5.8×10⁻³/Wm at thewavelength of 1.55 μm.

Also, if the land main optical communication system with a relay sectionof 100 km×6 span at bit rate 10 Gb/s is assumed, for example, thepolarization mode dispersion which is allowed in this opticalcommunication system is 10 ps or less. Therefore as mentioned above, ifthe polarization mode dispersion in the signal light propagation pathfrom the input end to the output end of the Raman amplifier is 1 ps orless in the signal light wavelength band, then the transmission qualityin this optical communication system is excellent.

(Optical Communication System)

The optical communication system according to the present inventionincludes an optical fiber transmission line where the signal light of aplurality of channels propagates, and a Raman amplifier having the abovementioned structure. Particularly to enable a long haul transmission,the optical communication system according to the present invention mayhave a plurality of Raman amplifiers having a structure similar to theRaman amplifier. The optical communication system according to thepresent invention can be modified in various ways to further improvesystem performance by improving the optical SN ratio.

As FIG. 11 shows, the optical communication system according to thefirst embodiment has a configuration to further improve the noisecharacteristic by causing induced Raman scattering in the optical fibertransmission line 10 at the input end side of the Raman amplifier, whichlocates at the most upstream side in the Raman amplifiers 1(2) (Ramanamplifier according to the present invention), which is provided at apredetermined position of the optical fiber transmission line 10 laidbetween the transmitting station 11 and the receiving station 12. FIG.11 is a diagram depicting the configuration of the first embodiment ofthe optical communication system according to the present invention.

Specifically, the optical communication system according to the firstembodiment comprises a pumping light source 13 (third pumping lightsource) for supplying new pumping light to the optical fibertransmission line 10 at the input end side, and optical coupler 14(third optical multiplexing structure) for guiding the pumping lightfrom the pumping light source 13 to the optical fiber transmission line10. In this way, by Raman-amplifying the signal light in advance, beforethe signal light is input to the Raman amplifiers 1(2) arranged on theoptical fiber transmission line 10, the noise characteristic isdramatically improved.

FIGS. 12A and 12B are diagrams depicting the configuration of the secondembodiment of the optical communication system according to the presentinvention. In the optical communication system according to the secondembodiment as well, a plurality of Raman amplifiers 1(2) (Ramanamplifiers according to the present invention) are arranged on theoptical fiber transmission line 10 laid between the transmitting station11 and the receiving station 12.

In particular, the optical communication system according to the secondembodiment comprises bypass transmission lines 15 a, 15 b and 15 c forsupplying pumping light which propagated at least a part of the Ramanamplification optical fiber 121 of the Raman amplifier, and opticalcouplers 14 a, 14 b and 14 c for guiding the pumping light whichpropagated the bypass transmission lines 15 a, 15 b and 15 c to theoptical fiber transmission line (fourth optical multiplexing structure)on the optical fiber transmission line at the input end side in eachRaman amplifier 1(2) to improve the noise characteristic of the entireoptical communication system.

Also to guide the pumping light from each Raman amplifier 1(2) to thebypass transmission lines 15 a, 15 b and 15 c, in this opticalcommunication system, an optical demultiplexer 16 for guiding the lightwhich propagated through the Raman amplification optical fiber 121 tothe bypass transmission lines 15 a, 15 b and 15 c respectively, and anoptical filter 17 for transmitting the pumping light out of the lightdemultiplexed by the optical demultiplexer 16, are provided in eachRaman amplifier 1(2), as shown in FIG. 12B.

As a consequence, according to the present invention, the dispersioncompensation section which implements the dispersion compensationfunction, and the optical amplification section which implements theRaman amplification function, are provided as independent devicecomposing elements. Therefore high design flexibility is obtained forboth the device design considering Raman amplification and the devicedesign considering dispersion compensation, without being restricted bythe respective design conditions. In particular, when the signal lightpropagation path in the Raman amplifier, excluding the dispersioncompensation section, has a cumulative chromatic dispersion whoseabsolute value is 5 ps/nm or less in the signal light wavelength band,then flexibility of device design considering both Raman amplificationand dispersion compensation further increases.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

1-49. (canceled) 50-52. (canceled)
 53. A Raman amplifier provided at apredetermined position of an optical fiber transmission line and havingan input end for entering signal light which propagates through saidoptical fiber transmission line and an output end for outputting saidRaman-amplified signal light, comprising: an optical amplificationsection provided between said input end and said output end, andincluding a Raman amplification optical fiber for Raman-amplifyingsignal light which enters through said input end by supplying pumpinglight into said Raman amplification optical fiber; wherein said Ramanamplifier further comprises a dispersion compensation section providedbetween said input end and said output end while being opticallyconnected to said Raman amplification optical fiber, said dispersioncompensation section compensating for a total chromatic dispersion ofsaid optical fiber transmission line positioned outside of said Ramanamplifier and said Raman amplification optical fiber positioned insideof said Raman amplifier, in a signal light wavelength band, and whereinsaid Raman amplification optical fiber has an effective area of 30 μm²or less in the pumping light wavelength, and a parameter g_(R) of2.3×10⁻¹⁴ m/W or more.
 54. A Raman amplifier provided at a predeterminedposition on an optical fiber transmission line and has an input end forentering signal light which propagates through said optical fibertransmission line and an output end for outputting said Raman-amplifiedsignal light, comprising: an optical amplification section which isinstalled between said input end and said output end, and includes aRaman amplification optical fiber for Raman-amplifying signal lightwhich enters through said input end by the supply of pumping light intosaid Raman amplification optical fiber; a dispersion compensationsection provided between said input end and said output end while beingoptically connected to said Raman amplification optical fiber, andcompensates for a total chromatic dispersion in the signal lightwavelength of said optical fiber transmission line positioned outside ofsaid Raman amplifier and said Raman amplification optical fiberpositioned inside of said Raman amplifier; and a pumping light sourcesupply system, comprising a first pumping light source, and suppliessaid pumping light to said Raman amplification optical fiber, and afirst optical multiplexing structure for guiding the pumping light fromsaid first pumping light source to said Raman amplification opticalfiber without passing through said dispersion compensating opticalfiber, and wherein said Raman amplification optical fiber has aneffective area of 30 μm² or less in the pumping light wavelength, and aparameter g_(R) of 2.3×10⁻¹⁴ m/W or more.